Method for forming a structural body and an apparatus for forming a structural body

ABSTRACT

A method for forming a structural body includes irradiating a substrate with light having a periodic intensity distribution and a wavelength within a wavelength region which allows the substrate to show opacity thereby forming a periodic structure causing optical diffraction on a surface of the substrate.

TECHNICAL FIELD

The invention relates to a method for forming a structural body and anapparatus for forming the structural body by executing this method.

BACKGROUND

In recent years, chemical color development using pigment substances hasbecome unacceptable in view of recycling properties and environmentalprotection. Under such circumstances, a structural color which developsa color utilizing a phenomenon such as diffraction and interference oflight by forming a fine structure has come to be an important techniquereplacing the chemical color development.

A structural color is generated, for example, by thin film interference,multilayer interference, light scattering phenomenon, diffractionlattice and photonic crystals.

It is difficult to cause such a structural color to be developedartificially. There are very few cases where structural colors aredeveloped on the industrial scale.

As one of these few cases, a method has been proposed in which a fineperiodic structure is formed by light irradiation, thereby developing astructural color.

As examples of the fine periodic structure formed by light irradiation,LIPS (Laser Induced Periodic Structures) (see Non-Patent Document 1, forexample) can be given, for example. The LIPS are fine periodicstructures which are formed by laser irradiation on the surface of amaterial in a self-organized manner.

As another example, a technology is disclosed in which a diffractionlattice is formed in the inside of glass by means of a femtosecond laser(see Non-Patent Document 2, for example).

Non-Patent Document 1: Sylvain Lazare: “Large scale excimer laserproduction of submicron periodic structures on polymer surface” AppliedSurface Science 69 (1993) 31-37, North-Holland

Non-Patent Document 2: Journal of Japanese Society of Laser Technology,Vol. 30, issue 2, Hideo Hosono and Kenichi Kawamura, “Interactionbetween Femtosecond Laser and Transparent Substance: Decoration of aTransparent Substance with a Periodic Nanostructure by a Single PulseInterference”, Japanese Society of Laser Technology, May 2005, pages7-12.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, to put a structural color into practical use, high colordevelopment efficiency and capability of being produced at low cost arerequirements. It is not easy to meet these three requirements.

For example, the technology disclosed in the Non-Patent Document 2 issince a lattice pattern can be formed only on one site by a single laserirradiation, an extremely long period of time is required to decorate alarge area, resulting in poor productivity.

In the case of commercial transactions, products must be strictlygenuine in order to keep the order of transactions, as well as toprotect consumers. However, the circulation of forgeries obtained byduplicating and imitating genuine products with the aim of takingadvantage of genuine products to gain illegal profits happens all toooften. Circulation of poor-quality forgeries not only decreases theprofits of the manufacturers of genuine products but also causesconsumers to distrust in the manufacturers of genuine products. As aresult, corporate images may deteriorate, and as a result, corporatepower of the brand may be impaired.

In order to avoid economical damage caused by the circulation offorgeries, many manufacturers take measures to distinguish genuineproducts from forgeries. As one of such measures, forgery protectionmarkings can be given. Examples of these include hard-to-duplicatemarkings which are attached to an object as the proof of a genuineproduct. Specific examples of these include a hologram seal in which arelief-type hologram image is formed (see JP-A-2006-059612 ((PatentDocument 1) and Japanese Patent No. 3546975 (Patent Document 2), forexample).

Since not only an advanced level of optical design technology isrequired but also a complicated material constitution and an expensivehologram original plate are also required, duplication of a hologramseal with the intent of forgery is difficult. Hologram seals are widelyused since they have forgery protection effects due to theabove-mentioned difficulty in duplication, the capability of beingdiscriminated at one glance due to its unique color tone, and easinessin handling which requires only physical attachment.

However, a hologram seal has the defect that it can be peeled offrelatively easily from an existing object and can be reused simply byattaching to another object. A hologram seal is attached as the proof ofa genuine product. Therefore, if the removed original seal is attachedto a forged product, it becomes impossible to verify that the product towhich the seal has been attached is a forgery.

The hologram seal disclosed in Patent Document 2 has a configuration inwhich a peelable layer is provided on an inner layer of the seal,whereby the seal itself is destroyed when the seal is peeled offintentionally. Due to the destruction, reuse of the peeled seal can beprevented. However, provision of a peelable layer makes the materialconstitution or manufacturing technique complicated, leading to anincrease in cost.

If the hologram seal is duplicated in a more elaborate fashion, toverify whether the attached hologram seal is real or not, fine detailedportions have to be compared carefully. Such comparison cannot be doneeasily by consumers.

The invention has been made in view of the above-mentionedcircumstances, and the object thereof is to provide a structural body, amethod for forming a structural body and an apparatus for forming astructural body.

Means for Solving the Problems

In order to solve the problem, the structural body of the invention hasa configuration in which a periodic structure causing opticaldiffraction is formed on part or the whole of the surface of thesubstrate.

By allowing the structural body to have such a configuration, since theperiodic structure is formed in the structural body itself, peeling theperiodic structure for reusing purposes is impossible. Therefore, it ispossible to prevent such an act of removing the periodic structure froman existing object like a hologram seal and attaching the removed sealto a forgery to allow the forgery to be circulated as a genuine product.

In addition, due to a configuration in which a periodic structure isformed on the surface of a substrate, the material constitution issimplified to prevent an increase in cost.

Furthermore, since a periodic structure is formed on the surface of asubstrate, a structural color is developed according to the periodicstructure.

In addition, the structural body of the invention may have aconfiguration in which the periodic structure formed on the surface ofthe substrate (substrate surface periodic structure) has a regulararrangement which develops a structural color.

If the structural body has such a configuration, the substrate surfaceperiodic structure may develop a structural color based on the regulararrangement.

The structural body of the invention may have a protective layer forprotecting a periodic structure formed on the substrate surface.

Due to such a configuration, the substrate surface periodic structurecan be prevented from being damaged.

In addition, in the structural body of the invention, the substratesurface periodic structure may be formed by light irradiation.

Due to such a configuration of the structural body, the substratesurface periodic structure may be formed by a relatively simplemanufacturing technique, whereby an increase in cost can be suppressed.

In the structural body of the invention, the periodic structure on thesurface of the substrate may be formed by light irradiation whichgenerating a periodic intensity distribution.

By this method for forming a structural body, due to the generation ofthe periodic intensity distribution, the substrate surface periodicstructure can be formed.

The method for forming a structural body of the invention comprisesirradiating by using an apparatus for forming a structural body thesubstrate with light having a wavelength within a wavelength regionwhich allows the substrate to show opacity, thereby allowing a periodicstructure causing optical diffraction to be formed on the surface of thesubstrate.

By this method for forming a structural body, by irradiating thesubstrate with light having a wavelength within a wavelength within awavelength region which allows the substrate to show opacity, asubstrate surface periodic structure can be formed on the surface of thesubstrate.

The method for forming a structural body of the invention comprisesirradiating the substrate with light which has a periodic intensitydistribution.

By this method for forming a structural body, the periodic structure canbe formed on the surface of the substrate by causing a periodic lightintensity distribution to be generated.

The apparatus for forming a structural body of the invention is astructural body-forming apparatus which irradiates light to thesubstrate. The apparatus comprises a laser oscillator which adjustsirradiation pulse number and/or laser output so that a periodicstructure causing optical diffraction can be formed in the interface ofthe cavity which is formed in the inside of the substrate.

By allowing the apparatus for forming a structural body to have such aconfiguration, by adjusting irradiation pulse number or laser output, aperiodic structure causing optical diffraction being formed on thesurface of the substrate.

Effects of the Invention

As mentioned above, according to the invention, a structural colorsatisfying the requirements for practical application on the industrialscale can be obtained.

Furthermore, the structural color of the invention can be effectivelyused for decorating plastic apparatuses or the like with a high degreeof recycling properties.

In addition, since the substrate surface periodic structure are formedin the substrate itself, it is impossible to remove them for reuse. As aresult, a damage caused by reusing it in a forgery, which occurs in thecase of forgery protection seals, can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view diagrammatically showing the structureof a structural body of this embodiment;

FIG. 2 a is a diagrammatical view showing the cross section of thestructural body in which a cavity is formed;

FIG. 2 b is a perspective view showing the appearance of the structuralbody in which a cavity is formed;

FIG. 2 c is a diagrammatical view of FIG. 2 b, showing a perspectiveview of an appearance of the structural body;

FIG. 2 d is an enlarged microscopic image of a cavity when thestructural body is viewed from top (direction A in FIG. 2 c);

FIG. 2 e is an enlarged microscopic image of a cavity when thestructural body is viewed from side (direction B in FIG. 2 c);

FIG. 3 a is a diagrammatical view showing the cross section of thestructural body in which a substrate surface periodic structure isformed;

FIG. 3 b is a perspective image showing the appearance of the structuralbody in which the substrate surface periodic structure is formed;

FIG. 3 c is an AFM image showing an enlarged view of the substratesurface periodic structure;

FIG. 4 is a view showing an arrangement of diffraction lattices forexplaining the Bragg's law;

FIG. 5 is a cross sectional view diagrammatically showing otherconfiguration of the structural body of this embodiment;

FIG. 6 is a schematic view showing the configuration of an apparatus forforming a structural body using a transmitting diffraction opticaldevice;

FIG. 7 is a diagrammatical perspective view showing the configuration ofan optical interference system of an apparatus for forming a structuralbody using a transmitting diffraction optical device;

FIG. 8 is a view showing an interference area of light to be irradiatedto the substrate;

FIG. 9 is a schematic view of an apparatus for forming a structural bodyusing a micro-lens array;

FIG. 10 is a diagrammatical perspective view showing the configurationof an optical interference system of the apparatus for forming astructural body using a micro-lens array;

FIG. 11 is a view showing light irradiation at a periodic lightintensity distribution;

FIG. 12 is a view showing a transmission spectrum of the substrate and aperiodic structure formed by the irradiation of light having a specificwavelength;

FIG. 13 is a view showing a method for generating a periodic lightintensity distribution, in which (a) is a view showing a method forforming a periodic light intensity distribution by the interference ofincident light and light reflected by the interface; (b) is a viewshowing a method for irradiating a monobeam of light having a periodiclight intensity distribution; and (c) is a view showing a method forgenerating a periodic light intensity distribution by the interferenceof a polybeam;

FIG. 14 is a view showing a method for forming a periodic structuralpattern when a transmitting diffraction optical device is used;

FIG. 15 is a view showing a method for forming a periodic structuralpattern when a micro lens-array is used;

FIG. 16 is a diagrammatical view showing a manner in which opticaldiffraction occurs;

FIG. 17 is a view showing a manner in which a cavity 12 is formed underthe following conditions:

YAG-THG (355 nm), 247 mJ/cm², 30 laser shots, a PET-INJ sheet as asample 30;

FIG. 18 is a view showing a manner in which a cavity 12 is formed underthe following conditions:

YAG-SHG (532 nm), 500 mJ/cm², 10 laser shots, an elongated PET sheet asa sample 30;

FIG. 19 shows plan observation images and diagrammatical cross sectionalviews of the cavity of the invention and the cavity of the ComparativeExample;

FIG. 20 is a table showing comparison between the laser irradiationconditions in Examples of the invention and the laser irradiationconditions in the Comparative Example; and

FIG. 21 is a graph showing comparison between laser irradiationconditions in the Example of the invention and laser irradiationconditions in the Comparative Example.

EXPLANATION OF NUMERALS

10. Structural body

11. Substrate

12. Cavity

13. Cavity interface periodic structure

14. Substrate surface

15. Substrate surface periodic structure

16. Protective layer

20. Apparatus for forming a structural body using a transmittingdiffraction optical device

21. Laser oscillator

23. Transmitting diffraction optical device

25. Mask

30. Apparatus for forming a structural body using a micro-lens array

35. Micro-lens array

36. Mask

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the structural body, the method for forming astructural body and the apparatus for forming a structural body will beexplained below with reference to the drawings.

In the embodiment, “decorating” implies coloring a package with achromatic color or shielding light in a specific wavelength region toprevent it from being penetrated into the inside of the package.

Structural Body

The embodiments of the structural body of the invention will beexplained with reference to FIGS. 1 to 3 c.

FIG. 1 is a cross-sectional view diagrammatically showing the structureof the structural body of this embodiment;

FIGS. 2 a to 2 e are views showing a cavity formed inside of thestructural body, FIG. 2 a is a diagrammatical view showing the crosssection of the structural body in which a cavity is formed; FIG. 2 b isa perspective view showing the appearance of the structural body inwhich a cavity is formed;

FIG. 2 c is a diagrammatical view of FIG. 2 b, showing a perspectiveview of an appearance of the structural body; FIG. 2 d is an enlargedmicroscopic image of a cavity when the structural body is viewed fromtop (direction A in FIG. 2 c); and FIG. 2 e is an enlarged microscopicimage of a cavity when the structural body is viewed from side(direction B in FIG. 2 c).

FIGS. 3 a to 3 c are views showing a periodic structure (substratesurface periodic structure) formed on the surface of the structural body(substrate surface), in which FIG. 3 a is a diagrammatical view showingthe cross section of the structural body in which the substrate surfaceperiodic structure is formed; FIG. 3 b is a perspective image showingthe appearance of the structural body in which the substrate surfaceperiodic structure is formed; and FIG. 3 c is an AFM image showing anenlarged view of the substrate surface periodic structure.

(Cavity, Cavity Interface Periodic Structure)

As shown in FIG. 1 and FIGS. 2 a to 2 e, a cavity 12 is formed in theinside of a substrate 11 of a structural body 10.

As shown in FIG. 2 a, each cavity 12 has almost a gong-like shape, andhas a diameter of about 40 μm at longest.

As for the cavity 12, the diameter direction of the gong-like structureis almost in parallel with the plane direction of the structural body10, and the thickness direction of the gong-like structure is almost inparallel with the thickness direction of the structural body 10.

As shown in FIG. 1 and FIGS. 2 a to 2 e, the periodic structure causingoptical diffraction (cavity interface periodic structure 13) is formedin the interface of the cavity 12. The periodic structure 13 has aregular arrangement which develops a structural color.

Here, the “periodic structure” means a structure in which a plurality ofpredetermined shapes is formed at an almost equal interval. In thecavity interface periodic structure 13 of this embodiment, as shown inFIGS. 2 a and 2 d, in the interface of a single cavity 12, a pluralityof convex portions and concave portions is formed vertically andlaterally at almost equal intervals. This periodic structure 13 isformed over the entire surface of the cavity 12. Therefore, the crosssection of the interface of the cavity 12 (cross section passing throughthe apex of the convex portion) has a wavy pattern, as shown in FIG. 2a. As apparent from the above, since the interval between the concaveportion and convex portion in the interface of the cavity 12 is close tothe wavelength of a visible light, a structural color is developed.

One period of the wavy pattern of the periodic structure 13 (thedistance between the apices of two adjacent convex portions) is about1.5 μm to 2.0 μm.

When light is incident on the periodic structure, diffraction occurs. Atthis time, a wavelength which is diffracted most strongly (a largeramount of photoenergy is diffracted) varies depending on the ratiobetween the wavelength and the lattice period. Generally, strongdiffraction occurs when the ratio of the light wavelength to the latticeperiod is about 1 or less.

The “regular arrangement which develops a structural color” as referredto herein means that the lattice period is close to the wavelength ofvisible light (about 400 nm to 700 nm), i.e., about 2.0 μm or less. Atthis time, since visible light is diffracted strongly, a structuralcolor is observed.

As shown in FIGS. 2 b and 2 c, the cavity 12 is formed at alight-irradiated part of the structural body 10.

A plurality of the cavity 12 is formed within the irradiated area(within the same plane).

These plural cavities 12 are formed at arbitrary positions laterallywithin the light-irradiated area. Furthermore, they are formed atdifferent depths from the light-irradiated surface.

Here, each cavity 12, which is formed in the same plane at differentdepths from the surface, functions as a diffraction lattice. Dependingon the light incident angle or the viewing angle, the structural body 10develops different colors (when cavities 12 are present in the sameplane, this plane is hereinafter referred to as the “diffractionplane”).

If the diffraction plane presents at different depths, i.e. presents ina plurality of layers, diffraction occurs at each of the diffractionplanes. At this time, emission is weak if the phase of the diffractionlight from each diffraction plane is not uniform. If the phase ofdiffraction light from each diffraction plane is uniform, strongemission can be obtained. Specifically, strong emission can be obtainedwith light having a wavelength satisfying the following Bragg'sreflection formula (formula 1):

mλ=2D(n2−sin 2θ)½  formula 1

In this formula 1, m is the order of diffraction, λ is a wavelength, Dis the interval of diffraction planes, n is the refractive index of amaterial and θ is a viewing angle assuming that the normal angle of thesample surface is 0°.

If the structural body 10, in which the diffraction planes are presentat intervals satisfying the formula 1, can be formed, the structuralbody 10 develops a color only by light having a specific wavelengthsatisfying the formula 1. Furthermore, if the number of diffractionplanes increases, stronger emission can be obtained due to an increasein the intensity of diffraction light.

As a material which develops a color according to the Bragg's Law,photonic crystals are known in which periodic structures arethree-dimensionally formed.

The perspective image in FIG. 2 b and the microscopic images in FIGS. 2d and 2 e show the structural body 10 obtained by using a PET 3×3elongated sheet. The raw material for the structural body 10 is,however, not limited thereto. Any material may be used insofar as thecavity 12 is formed in its inside by light irradiation.

(Substrate Surface Periodic Structure)

As shown in FIG. 1 and FIGS. 3 a to 3 c, in the surface of the substrate11 of the structural body 10 (substrate surface 14), a periodicstructure causing optical diffraction (substrate surface periodicstructure 15) is formed. This periodic structure 15 has a regulararrangement developing a structural color. The “regular arrangementdeveloping a structural color” as referred to herein mean a state inwhich the lattice period is close to the visible light wavelength (about400 to 700 nm).

Here, the periodic structure is as defined above. In the substratesurface periodic structure 15 of this embodiment, as shown in FIGS. 3 aand 3 c, a plurality of concave and convex portions is formed at almostequal intervals on a substrate surface 14. This periodic structure 15 isformed on the entire substrate surface 14. Therefore, the cross sectionof the substrate surface 14 (cross section passing through the apices ofthe convex portions) has a wavy pattern as shown in FIG. 3 a. Astructural color is developed since the interval between convex portionsis close to a visible light wavelength.

One period of the wavy pattern of the substrate periodic structure 15(distance between the apices of adjacent convex portions) is about 1.0μm.

The substrate surface periodic structure 15 is formed at alight-irradiated portion of the substrate surface 14. Therefore, asshown in FIG. 3 b, in order to form a periodic structure over the entiresurface of the substrate surface 14, it is preferred that a plurality ofportions of the substrate surface 14 be irradiated with light such thatno gaps are made depending on the area of the substrate surface 14.

The images of FIGS. 3 b and 3 c show the structural body 10 obtained byusing a PET 3×3 elongated sheet. The raw material of the structural body10 is, however, not limited thereto. Any material may be used insofar asthe substrate surface periodic structure 15 is formed in its inside bylight irradiation.

As mentioned above, the structural body 10 of this embodiment has aconfiguration in which the cavity 12 with a fine periodic structurebeing formed in the interface thereof is formed within the substrate 11,and the substrate surface 14 has a fine periodic structure. By allowinga structural color to be developed from the periodic structures 13 and15 by an optical phenomenon such as diffraction and interference,whereby a marking is formed.

Here, the “marking” is defined by an area where portions which develop astructural color and/or diffraction light are uniformly formed, or agraphic, a letter or the like which is formed as a result of adequatearrangement of portions which develop a structural color and/ordiffraction light.

By forming the cavity interface periodic structure 13 and the substratesurface periodic structure 15 in the substrate itself, it is impossibleto remove them for reuse purposes, such as attaching to other objects.Also, it is impossible to delete or alter the periodic structures 13 and15.

(Substrate)

The substrate 11 is an element which serves as the base of thestructural body 10.

In the substrate 11, high-molecular compounds such as polystyrene,polyethylene, polypropylene, polycarbonate, nylon resins, acrylicresins, vinyl chloride resins and phenol resins, optical glass such asBK7 and quartz and soda glass may be used as raw materials. It is alsopossible to use polyester compounds such as polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT)and polytrimethylene terephthalate (PTT) as raw materials.

The raw materials for the substrate 11 are not limited to the materialsmentioned above, and suitable known materials may also be used insofaras the materials can allow the cavity 12, the cavity interface periodicstructure 13 and the substrate surface periodic structure 15 to beformed by light irradiation.

(Protective Layer)

As shown in FIG. 5, a protective layer 16 may be provided in thestructural body 10.

The protective layer 16 is a protective film to avoid such anunfavorable phenomenon that the substrate surface periodic structure 15is damaged to become unable to develop a structural color.

The protective layer 16 may be formed of acrylamino, polyesteramino,polyesteracrylamino or the like, for example.

Apparatus for Forming a Structural Body

Next, an explanation is made on an apparatus for forming a structuralbody. There are two types of apparatuses for forming a structural body,i.e. one which has a transmitting diffraction optical device and onewhich has a micro-lens array.

(Apparatus for Forming a Structural Body Using a TransmittingDiffraction Optical Device)

First, with reference to FIGS. 6 to 8, an explanation will be made on anapparatus for forming a structural body utilizing a transmittingdiffraction optical device.

FIG. 6 is a schematic view showing a structure of the apparatus forforming the structural body, FIG. 7 is a diagrammatical perspective viewshowing a constitution of an interference optical system of theapparatus for forming the structural body, and FIG. 8 is a view showingthe manner in which light beams intersect and interfere around thesubstrate (structural body) to form a high intensity area.

The apparatus for forming the structural body 20 is an apparatus forforming the cavity interface periodic structure 13 or the substratesurface periodic structure 15 in the substrate 11 (structural body 10).As shown in FIGS. 6 and 7, the apparatus is provided with a laseroscillator 21, a mirror 22, a transmitting diffraction optical device23, a convex lens 24, a mask 25 and a concave lens 26.

Here, the laser oscillator 21 is an apparatus which outputs a laserbeam. Examples include a YAG laser which performs Q-switch oscillation,a YVO₄ laser, a YLF laser and a femtosecond laser such as a Ti:sapphirelaser. These pulse lasers have repeated frequency of several Hz toseveral tens of kHz. The energy accumulated during these repeatedperiods is released at an extremely short time interval such as severalfemtoseconds to several tens of nanoseconds. Therefore, a high peakpower can be obtained efficiently from a small amount of input energy.

To form the periodic structure 13 in the inside of the substrate 11, 5to 20 laser shots will suffice. Such laser shots require about severalseconds, realizing high-speed production. As a result, productsdeveloping a structural color can be produced at a low cost. Inaddition, a further shortening of production time can be attained byusing a light source having a high repeated frequency.

The laser oscillator 21 has a function of adjusting the irradiationpulse number. The laser oscillator 21 can control the energy density(fluence: energy per area irradiated with one pulse) by adjusting theoutputs of the laser.

Control of the energy density can be realized not only by adjusting thelaser output of the laser oscillator 21 but also by changing the laserbeam diameter with the laser output being the same.

To allow the periodic structure to be formed in the inside of theirradiated material, it is required that the irradiation light bepenetrated into the inside of the material. To prevent the irradiationlight from being absorbed on the polar surface, it is desirable to uselight with a wavelength exhibiting an appropriate transmittance at theirradiated material.

A mirror 22 reflects laser light which has been output from the laseroscillator 21. In FIG. 6, two mirrors 22 are provided. The number of themirror 22 is, however, not limited to two, and an arbitrary number ofmirrors may be provided.

The transmitting diffraction optical device 23 is a transmitting opticaldevice which causes diffraction since fine convex and concave portionsare periodically formed on the surface thereof, and divides laser lightinto a plurality of light beams.

As the convex lens 24, a synthetic quartz flat convex lens having afocal distance of 200 mm can be used, for example. In this case, theconvex lens 24 is placed at a position 200 mm away from the transmittingdiffraction optical device 23. The convex lens 24 allows the pluralityof light beams which have been divided by the transmitting diffractionoptical device 23 to pass therethrough.

The mask 25 is placed at a position at which the light beams which havebeen passed through the convex lens 24 focus, and hides light beamswhich are not required for interference but passes through light beamswhich are required for interference.

As the convex lens 26, a synthetic quartz flat convex lens having afocal distance of 100 mm can be used, for example. The convex lens 26condenses light beams which have passed through the mask 25, and causesthe light beams to intersect and interfere. The interference region hasa high intensity distribution, as shown in FIG. 8. The substrate 11 isirradiated with this region.

Here, the period d of the interference fringe can be expressed by thefollowing formula:

d=λ/(2 sin(θ/2)  formula 2

λ: light wavelength, θ: intersecting angle of light beam

As for the relationship between the division of the laser beam by meansof the transmitting diffraction optical device 23 and the selection ofthe laser beam by means of the mask 25, an explanation will be made inthe “Method for forming a periodic structural pattern by using atransmitting diffraction optical device” in the “Method for forming aperiodic structural pattern”, which is given later.

(Apparatus for Forming a Structural Body by Using a Micro-lens Array)

Next, an apparatus for forming a structural body will be explained withreference to FIGS. 9 and 10.

FIG. 9 is a schematic view explaining a configuration of the apparatusfor forming a structural body. FIG. 10 is a diagrammatic perspectiveview of the optical interference system of the apparatus for forming astructural body.

As shown in FIGS. 9 and 10, the apparatus for forming the structuralbody 30 has a laser oscillator 31, a mirror 32, a concave lens 33, aconvex lens 34, a micro-lens array 35, a mask 36 and a convex lens 37.

Here, the laser oscillator 31 has the same function as that of the laseroscillator 21 of the apparatus 20 for forming a structural body shown inFIG. 6.

The mirror 32 reflects laser light which has been output from the laseroscillator 31. In FIG. 9, two mirrors 32 are provided. The number of themirror 32 is, however, not limited to two, and an arbitrary number ofmirrors may be provided.

The concave lens 33 enlarges the diameter of the laser beam. The convexlens 34 adjusts the enlarged laser beam diameter to a desired diameter.

The micro-lens array 35 is an optical device in which fine convex lensesare arranged in a checkerboard pattern, and divides the laser light intoa plurality of beams.

The mask 36 selects some of the divided light beams. The convex lens 37collects light beams which have been selected by the mask 36.

As for the relationship between the division of the laser beam by meansof the micro-lens array 35 and the selection of the laser beam by meansof the mask 36, an explanation will be given in the “Method for forminga periodic structural pattern by using a transmitting diffractionoptical device” in the “Method for forming a periodic structural patternby using a micro-lens array”, which is given later.

Method for Forming a Structural Body

Next, an explanation will be made on the method for forming thestructural body of this embodiment with reference to FIGS. 11 and 12.

FIG. 11 is a view showing the substrate being irradiated with lighthaving a periodic intensity distribution. FIG. 12 is a view showing therelationship between the wavelength of light to be irradiated and theperiodic structure formed by this irradiation.

As shown in FIG. 11, by irradiating the substrate 11 with light 1 havinga periodic intensity strength distribution, the cavity 12 having theperiodic structure 13 in the interface thereof is formed in the insideof the substrate 11. At this time, the fine periodic structure 13 isformed with the same period as that of the periodic intensitydistribution.

On the other hand, by irradiating the substrate 11 with light 2 whichhas a periodic intensity distribution, the periodic structure 15 isformed on the substrate surface 14. At this time, the fine periodicstructure 15 is formed with the same period as that of the periodicintensity distribution.

Here, the light 1 is light having a wavelength within a wavelengthregion which allows the substrate 11 to show transparency. The light 2is light having a wavelength within a wavelength region which allows thesubstrate 11 to show opacity. This means that, in the method for formingthe structural body 10, dependency of the optical characteristics of thesubstrate 11 on the light wavelength is utilized.

For light with a specific wavelength, the substrate 11 has any of thefollowing properties: transparency with a transmittance of 70% or more,semi-transparency with a transmittance of less than 70%, and opacitywith a transmittance of less than 10%. When the substrate 11 showstransparency for a certain wavelength, the light penetrates into theinside of the substrate. If the substrate 11 shows opacity for a certainwavelength, the light enters only in the proximity of the surface of thesubstrate.

Specifically, as shown in FIG. 12, the substrate 11 is irradiated withlight having a wavelength of about 330 nm or more (355 nm, for example),which is the light within a wavelength region allowing the substrate 11to show transparency, whereby the cavity 12 and the cavity interfaceperiodic structure 13 are formed. The substrate 11 is irradiated withlight having a wavelength of about 310 nm or less (266 nm, for example),which is the light having a wavelength within a wavelength regionallowing the substrate 11 to show opacity, whereby the substrate surfaceperiodic structure 15 is formed.

There is no specific order of irradiation between the light having awavelength within a wavelength region which allows the substrate 11 toshow transparency and the light having a wavelength within a wavelengthregion which allows the substrate 11 to show opacity. However, it ispreferred that the substrate 11 be irradiated with the light having awavelength within a wavelength region which allows the substrate 11 toshow transparency at first, followed by the irradiation with the lighthaving a wavelength within a wavelength region which allows thesubstrate 11 to show opacity.

The reason for this is to prevent the following unfavorable phenomenonfrom occurring. Specifically, if the substrate surface periodicstructure 15 has been formed in advance, the periodic optical intensitydistribution is disarranged when forming the cavity interface periodicstructure 13. As a result, a well-arranged periodic structure is notformed, and development of a structural color is deteriorated.

Irradiation of light having a periodic intensity distribution can beperformed by the irradiation of parallel rays by means of a mask withperiodically arranged openings or by the irradiation of the interferenceregion obtained by intersecting a plurality of parallel rays.

Generally, a single pulse laser shot will suffice to form the cavityinterface periodic structure 13. About 5 to 20 pulse laser shots willsuffice to form the substrate surface periodic structure 15. This takesonly several seconds. In addition, by using a laser light source havinga large pulse repetition frequency, a further shortening of time can berealized.

As the substrate surface periodic structure, LIPS (Laser InducedPeriodic Structures) may be given other than those mentioned above.

This is a fine periodic structure formed in a self-organized manner byspontaneously generating a periodic intensity distribution on thesurface of a material by laser irradiation. In addition, it is alsopossible to use a hot stamp method in which a pattern having a periodicstructure is pushed to the substrate after heating or a pattern having aperiodic structure is pushed to the heated substrate.

The above-mentioned LIPS are disclosed in the following Non-PatentDocument.

Non-Patent Document: Sylvain Lazare: “Large scale excimer laserproduction of submicron periodic structures on polymer surface”, AppliedSurface Science 69 (1993), pages 31 to 37, North Holland

The periodic light intensity distribution for forming the cavity 12 orthe cavity interface periodic structure 13 can be formed by thefollowing three methods, for example.

As the first method, as shown in FIG. 13( a), a periodic light intensitydistribution is generated by the interference of incident light andlight reflected at the interface.

As the second method, as shown in FIG. 13( b), a periodic lightintensity distribution is generated by the irradiation of a single beamhaving a periodic light intensity distribution.

As the third method, as shown in FIG. 13( c), a periodic light intensitydistribution is generated by the interference of multiple beams.

The above-mentioned irradiation of light having a periodic intensitydistribution can be formed by the irradiation of parallel rays through amask having periodically-arranged openings or by the irradiation of aninterference area obtained by intersecting a plurality of light beams.

Method for Forming a Periodic Structural Pattern

The method for forming a periodic structural pattern when thetransmitting diffraction optical device or the micro-lens array isfunctioned as an optical control device is explained with reference toFIGS. 14 and 15.

When light is incident on the periodic structure, diffraction lightappears in the direction of periodicity.

The optical control device is defined as a device which controls thepattern (direction, angle and wavelength) of diffraction light. Thiscontrol is performed by adjusting the pattern (the number of the axis ofperiodicity, the angle of periodicity and the periodicity of lattice) ofthe periodic structure. As a result, optical information (diffractionpattern) can be recorded as a sort of bar code utilizing diffraction.

The pattern of the periodic structure of the structural body is the sameas that of the periodic intensity distribution of light to beirradiated. That is, by using different periodic intensity distributionpatterns, it is possible to create various patterns of periodicstructures.

The pattern of the periodic intensity distribution can be changed bychanging the position of an opening, if parallel rays are irradiatedthrough a mask having an opening. The pattern of the periodic intensitydistribution can be changed by changing the number, intersectingdirection, intersecting angle and wavelength of light beams ifirradiating with an interference area which is obtained by intersectinga plurality of light beams.

In the former method, of the parallel rays incident on the mask, onlylight near the openings passes through the mask. The light then advancesstraightforwardly, and is incident on an object. Therefore, the patternof the periodic intensity distribution coincides with the arrangement ofthe mask openings.

In the case where irradiation is performed with an interference area, asthe method for obtaining a plurality of beams by dividing a single beam,two methods are available; i.e. the use of a transmitting diffractionoptical device and the use of a micro-lens array.

Each of these methods is explained below.

(Method for Forming a Periodic Structural Pattern by Using aTransmitting Diffraction Optical Device)

When a transmitting diffraction optical device is used, a laser beam isincident on a transmitting diffraction optical device. The incidentlaser beam is divided into two beams, i.e. a beam which advancesstraightforwardly and a beam which diffracts. The light beam to bediffracted appears in the direction of periodicity.

From the relationship between the division of the light beam by thetransmitting diffraction optical device and the opening of the mask, thepattern of the periodic structure shown in FIGS. 14( a) and (b) can beobtained.

Here, as shown in FIG. 14( a), the laser light is divided into 3×3 laserbeams. Of these, if a pair of laser beams which are positioneddiagonally pass in correspondence with the opening of the mask, aslanted striped periodic structural pattern is formed.

As shown in FIG. 14( b), if two pairs of laser beams positioneddiagonally pass in correspondence with the opening of the mask, adot-like periodic structural pattern is formed.

As is apparent from the above, the periodic structural pattern variesdepending on the combination of the light-dividing pattern of thetransmitting diffraction optical device and the pattern of maskopenings. As a result, optical information can be recorded as a sort ofbarcode utilizing diffraction.

(Method for Forming a Periodic Structural Pattern by Using a Micro-lensArray)

The method using a micro-lens array is a method which can be compared togelidium jelly (tokoroten)-making. Specifically, a laser beam is allowedto be incident on the micro-lens array as in the case where the gelidiumjelly is pushed out by means of a gelidium jelly-making instrument. As aresult, the laser light is divided into a plurality of laser beams in acheckerboard pattern.

For example, from the relationship between the division of the lightbeam by the micro-lens and the mask opening, the periodic structuralpattern to be recorded as the cavity interface periodic structure 13 orthe substrate surface periodic structure 15 is as shown in FIGS. 15( a)to (c).

Here, as shown in FIG. 15( a), the laser light is divided into 36 (6×6)laser beams. Of these, if the laser beam positioned at the intersectionof column 2 and row 2 and the laser beam positioned at the intersectionof column 5 and row 5 pass in correspondence with the mask opening, aperiodic structural pattern with an oblique striped pattern is recorded.At this time, the number of the axis of periodicity is 1, and thedirection of periodicity is in perpendicular to the direction in whichone of the stripes extends.

Furthermore, as shown in FIG. 15( b), the laser light is divided into 25(5×5) laser beams by the micro-lens array. Of these, if the laser beampositioned at the intersection of column 2 and row 2, the laser beampositioned at the intersection of column 2 and row 4, the laser beampositioned at the intersection of column 4 and row 2 and the laser beampositioned at the intersection of column 4 and row 4, respectively, passin correspondence with the mask opening, a fine dot periodic structuralpattern is recorded. At this time, the number of periodicity axis isfour, and the direction of periodicity is eight, specifically, theupward periodicity, the downward periodicity, the rightward periodicity,the leftward periodicity and the two oblique periodicities, with oneconcave portion being as the center.

Furthermore, as shown in FIG. 15( c), for example, the laser light isdivided into 25 (5×5) laser beams by the micro-laser. Of these, if thelaser beam positioned at the intersection of column 2 and row 2, thelaser beam positioned at the intersection of column 2 and row 4, thelaser beam positioned at the intersection of column 3 and row 3, thelaser beam positioned at the intersection of column 4 and row 2, and thelaser beam positioned at the intersection of column 4 and row 4 pass incorrespondence with the mask opening, coarse dot-like periodicstructural pattern is recorded. At this time, the number of axis ofperiodicity is 4, and the direction of periodicity is eight,specifically, the upward periodicity, the downward periodicity, therightward periodicity, the leftward periodicity and the two obliqueperiodicities, with one concave portion being as the center.

As apparent from the above, the periodic structural pattern variesdepending on the combination of the light division pattern of themicro-lens array and the mask opening pattern. As a result, opticalinformation can be recorded utilizing diffraction as a sort of bar code.

Method for Reading a Marking

Then, a method for reading a marking is explained.

Here, the “method for reading a structural color or diffraction light”is explained.

(Method for Reading a Structural Color or Diffraction Light)

As shown in FIG. 16, if light is incident on a surface having aone-dimensional periodic structure (e.g. a reed screen-like structure)or a two-dimensional periodic structure (e.g. a lattice structure), thelight is divided at different angles (scattered) for differentwavelengths. This phenomenon is called “diffraction”. The “diffractionlight” means all of the light scattered by diffraction. Here, “0-orderlight” means light remained without being diffracted.

If natural light (sun light, for example) is used as a light source,light in the UV region, visible region and infrared region is diffractedat different angles. At this time, diffracted light in the visibleregion (same as the light in other wavelength regions) is diffracted atdifferent angles for each wavelength (that is, separated). Therefore, adifferent color is seen depending on the position at which diffractionlight in the visible region is visually observed. The “structural color”as referred to herein means diffracted light in the visible region or acolor observed by the diffraction light in the visible region.

Here, the visible region is a wavelength region which can be observed byhuman eyes, and has a wavelength of about 400 to 700 nm. A wavelengthregion shorter than the visible region is referred to as the “UV region”and a wavelength region longer than the visible region is referred to asthe “infrared region”, and these wavelength regions cannot be observedby human eyes.

An angle β at which light having a specific wavelength is diffracted canbe obtained by the following formula using a wavelength λ, an incidentlight a and a period d of the periodic structure:

d(sin α±sin β)=mλ  formula 3

Here, m indicates the order of diffraction.

For example, when a monochromatic spotlight like a laser beam isincident on a marking, spot-like diffraction light appears only at aspecific angle. If a surface having a marking is uniformly irradiatedwith monochromatic optical diffraction occurs only at the portion of themarking. Diffraction light appears in the same shape as that of themarking only at the specific angle.

Here, the marking indicates a graphic, a letter or the like formed byadequately arranging areas in which diffraction-causing portions areuniformly formed or by adequately arranging diffraction-causingportions.

From the above, “reading diffraction light” or “reading a structuralcolor” indicates any of the following.

-   i. Detecting diffraction light at a diffraction angle obtained from    the formula 3 by using a known lattice period when known    monochromatic light is incident at a specific angle;-   ii. Measuring an angle at which diffraction light is detected when    known monochromatic light is incident at a specific angle;-   iii. At a diffraction angle obtained from the formula 3 using a    known lattice period, measuring an angle capable of detecting    diffraction light at which known monochromic light is incident;-   iv. When light including a plurality of wavelengths is incident at a    specific angle, detecting diffraction light having a specific    wavelength at a diffraction angle obtained from the formula 3 by    using a known lattice period;-   v. When light including a plurality of wavelengths is incident,    measuring an angle capable of detecting diffraction light having a    specific wavelength; and-   vi. At a diffraction angle obtained from the formula 3 using a known    lattice period, measuring an angle capable of detecting diffraction    light having a specific wavelength at which light including a    plurality of wavelengths is incident.

If spot light is incident, the above-mentioned diffraction light is inthe form of a spot, and if uniform light is incident on a surfaceincluding a marking, the diffraction light has the same shape as that ofthe marking.

Here, the “reading diffraction light” and the “reading a structuralcolor” differ as follows.

“Reading diffraction light” is one of the above-mentioned methodsapplied for diffraction light in any of the UV region, the visibleregion and the infrared region. “Reading a structural color” is one ofthe above-mentioned methods applied for diffraction light in the visibleregion.

In the case of the “reading diffraction light”, diffraction light in thevisible region is detected by human eyes in addition to a light receiver(detector). On the other hand, in the case of the “reading a structuralcolor”, the structural color is detected by both human eyes and a lightreceiver.

By observing whether diffraction light can be detected or not by theabove-mentioned “reading” method or by observing the measured angle, itcan be judged whether or not the value of the lattice period is the onewhich has already been known.

Irradiating a surface including a marking with natural light, andobserving, at an angle of visible observation, a different color and theshape of a marking correspond to iv. as mentioned above.

Examples of Formation of a Structural Body

Examples of the formation of a structural body are explained below.

EXAMPLE 1

By passing the light beam emitted from a Q-switch pulse YAG laserthrough a transmitting diffraction optical device, the light beam wasdivided into a plurality of light beams.

Each of the light beams was caused to pass through a synthetic quartzflat convex lens with a focal length of 200 mm, which was placed 200 mmaway from the transmitting diffraction optical device. At a positionwhere the light beams which had passed the lens was focused, unnecessarylight beams were hidden by a mask, thereby allowing only necessary lightbeams to pass through. The light beam which had passed through wasfocused by means of a synthetic quartz convex lens having a focal lengthof 100 mm. The light beam was caused to intersect and interfere. Abiaxial elongated PET sheet was irradiated with the interference area.In advance, irradiation was performed with a laser beam having awavelength of 355 nm (transmission for PET sheet of 83%).

Then, the wavelength was switched to 266 nm (transmittance for PETsheet: 0.3%), and irradiation was performed. The two-dimensional latticeperiod of the diffraction optical device was 6 μm. The pulse YAG laserhas the following specification:

Pulse width: 5 ns

Repeated frequency: 10 Hz

When the laser wavelength was 355 nm, the average power of irradiatedlight was 1.35 W. Since the irradiated area was 4.5 mmφ, the light powerdensity per unit area was 1 W/cm².

As a result of 25 laser shots, cavities were generated in the inside ofthe elongated PET sheet, and in the interface of the cavity,two-dimensional periodic structure with a period of 1.5 μm was formed. Astructural color caused by this structure could be observed.

Similarly, at a laser wavelength of 266 nm, the average power of theirradiated light was 240 mW. Since the irradiated area was 4.5 mmφ, thedensity of light power per unit area was 179 mW/cm².

As a result of a single laser shot, a two-dimensional periodic structurewith a period of 1 μm was formed on the outside surface of the elongatedPET sheet. A structural color caused by this structure was observed.

If the cavity 12 was formed under the conditions of Example 1, amicroscopic observation as shown in FIGS. 2 d and 2 e was obtained. Theconditions under which the cavity 12 was formed are not those describedin Example 1.

For example, the cavity 12 formed by using a PET-INJ sheet as a sample30 under the condition of YAG-THG (355 nm), 247 mJ/cm² and 30 lasershots had a shape shown in FIG. 17.

Furthermore, the cavity 12 formed by using a PET-INJ sheet as the sample30 under the condition of YAG-SHG (532 nm), 500 mJ/cm² and 10 shots hada shape shown in FIG. 18.

EXAMPLE 2

A biaxial elongated PET sheet was irradiated with a laser having awavelength of 266 nm in the same manner as in Example 1. A structuralcolor was observed by the formed two-dimensional periodic structure.

To this periodic structure, an ester oil (CAS-Nr: 195371-10-9) having arefractive index of 1.518 which is close to that of the elongated PETsheet (1.64) was applied. As a result, the sheet was observed like theoriginal transparent elongated PET sheet since no structural color wasobserved.

Comparison with Comparative Examples

Next, comparison between the structural body of this embodiment and thestructural body of the comparative example is explained with referenceto FIGS. 19 to 21.

(Comparison Between the Cavity of This Embodiment and Pores of theComparative Example)

JP-A-2004-359344 discloses a technology of developing a color by forminga plurality of fine pores (“Plastic package and a method for decoratingthe same”, hereinafter abbreviated as “Comparative Example 1”).

The structural body of Comparative Example 1 and the structural body ofthis embodiment are compared to clarify the difference.

-   (1) Structure to be Formed is Different

In Comparative Example 1, the pores formed by gasification due to heatgeneration are arranged according to the periodic light intensitydistribution.

On the other hand, in the invention, cavities are formed by aphotochemical reaction between laser light and a material, and aperiodic structure is formed in the interface according to a periodiclight intensity distribution. The cavities are not necessarily formedperiodically.

The structure of the pore of Comparative Example 1 and the structure ofthe cavity of the invention are shown in FIG. 19.

As shown in FIG. 19, each of the pores of Comparative Example 1 isformed in an almost spherical shape, and a plurality of pores wasarranged in a periodic arrangement structure, enabling decoration to beenabled.

On the contrary, in the invention (Example), a periodic structure isformed in the interface of the cavity, and this periodic structure has aregular arrangement which enables decoration.

-   (2) Sample is Different

In Comparative Example 1, since heat generation by laser lightirradiation is utilized, a light-absorbing heat generator was kneaded toefficiently convert light energy to heat energy.

In contrast, in the invention, use of a specific additive is notnecessary.

The difference between these samples is shown in FIG. 20. As shown inFIG. 20, in Comparative Example 1, an injection-molded PET sheet(thickness: 1.5 mm) to which a light-absorbing heat generator had beenkneaded was used as a sample. The transmittance at a wavelength of 355nm was 0%. At this time, the light was penetrated into the inside of thematerial, and the transmittance was 0% as a result of exponential decay.In contrast, in the invention (Example), a biaxial elongated PET sheet(thickness: 150 μm) was used as a sample, and the transmittance was 83%.

-   (3) Formation Principle is Different

The method in Comparative Example 1 is a method in which heat generationby laser beam irradiation is utilized. In contrast, in the invention, astructural body is formed by using laser light irradiation, withoutusing heat generation.

Here, the laser light irradiation conditions in the Example of theinvention and the laser light irradiation conditions of ComparativeExample 1, which is described in the background of the JP-A-2004-359344as the third embodiment, are shown in FIG. 21. In FIG. 21, the axis ofabscissa shows fluence (photoenergy per area irradiated with one pulse)and the axis of ordinates shows the irradiation pulse number.

In FIG. 21, ∘ indicates the conditions under which the cavity 11 isformed in the structural body of the invention, x indicates theconditions under which the cavity 11 is not formed and □ indicates theconditions under which the pore is formed in Comparative Example 1.

As shown in FIG. 21, the method for forming a structural body of theinvention needs only a smaller irradiation pulse number as compared withComparative Example 1. This is because heat generation is not used informing the structural body.

It is also understood that the forming method of the invention needs asmaller fluence as compared with Comparative Example 1.

As mentioned hereinabove, according to the structural body, the methodfor forming a structural body and the apparatus for forming a structuralbody, a cavity is formed in the inside of a material to be irradiated,and a periodic structure having a periodic intensity distribution ofirradiation light is formed in the interface of the cavity.

As a result, the structural body may have resistance to scars or spotswhich cause color developability to be lowered. Furthermore, by formingthe cavity three-dimensionally, color developability can besignificantly improved. In addition, due to the shortened formationtime, productivity can be enhanced. As mentioned above, while satisfyingthe requirements for practically putting a structural color intopractical use, decoration of a material is enabled with a high degree ofrecycling properties.

Since no chemical colorant such as a pigment and a dye is used, thestructural body of the invention is a material which imposes a lesserburden on the environment due to the decrease in use of the amount ofchemicals.

Furthermore, in the structural body of the invention, decoration is madeby a periodic structure formed in the interface of the cavity.Therefore, the decoration can be deleted easily by heating and kneading(re-pelletizing) at the time of recycling. Therefore, although thestructural body is decorated when used as a container, it becomescolorless and transparent after re-treatment, and hence, exhibits a highdegree of recycling properties.

In addition, unlike chemical color development, since a vivid color tonehaving a deep shade and gloss can be obtained, differentiation ofproducts can be performed effectively by decoration.

Furthermore, by using a structural color and/or diffraction light due tothe periodic structure formed in the structural body itself, it ispossible to verify whether an object is genuine or not. In addition,since the periodic structure is formed in the inside of the structuralbody, it is impossible to remove it for application to other objects.Therefore, unlike the conventional hologram seal, the act of removingand reusing by attaching to a forgery product with the aim of allowingthe forgery product to be put on the market as a genuine product can beprevented.

In addition, since the periodic structure is formed on a singlesubstrate, it is not required to form a complicated structure bycombining a plurality of materials. Therefore, constitution of thematerial can be simplified, thereby eliminating an increase of the rawmaterial cost.

Furthermore, since the periodic structure can be formed by lightirradiation, the structural body can be formed at a low cost without theneed of introducing an expensive apparatus for forming the structuralbody.

Hereinabove, the preferred embodiments of the structural body, themethod for forming a structural body and the apparatus for forming astructural body are explained. However, the structural body, the methodfor forming a structural body and the apparatus for forming a structuralbody are not limited to the above-mentioned embodiments. It is needlessto say that various modifications are possible within the scope of theinvention.

For example, in the above-mentioned embodiments, PET was used as thesample. PET can be used, for example, as a material for containers,cups, pouches, trays, and tubular containers. In this case, theinvention can be provided as a technology of decorating plasticcontainers with a high degree of recycling properties.

In the above-mentioned embodiment, the substrate surface periodicstructure is formed only on one surface of the substrate. However, thesubstrate surface periodic structure is formed not only on a singlesurface, but may be formed on two or more surfaces. For example, thesubstrate surface periodic structure may be formed on both sides of asheet element, or on two or more sides of a cubic element.

In the above-mentioned embodiment, a forgery-protection marking is givenas the application of the invention. The application of the invention isnot limited to the forgery-protection marking. For example, theinvention can be applied to decoration, utilizing the effect in which aninternal marking appears by the above-mentioned reading method.

INDUSTRIAL APPLICABILITY

The invention is provided for realizing development of a structuralcolor on the industrial scale. Therefore, the invention can be appliedto a field where a material requiring decoration is used.

1. A method for forming a structural body, comprising: irradiating asubstrate with light having a wavelength within a wavelength regionwhich allows the substrate to show opacity thereby forming a periodicstructure causing optical diffraction on a surface of the substrate. 2.The method for forming a structural body according to claim 1, whereinthe light to be irradiated to the substrate has a periodic intensitydistribution.
 3. The method for forming a structural body according toclaim 1, wherein the periodic structure formed on the surface of thesubstrate has a regular arrangement which develops a structural color.4. The method for forming a structural body according to claim 2,wherein the periodic structure formed on the surface of the substratehas a regular arrangement which develops a structural color.
 5. Themethod for forming a structural body according to claim 1, wherein theperiodic structure causing optical diffraction is formed on a part or awhole of the surface of the substrate.
 6. The method for forming astructural body according to claim 2, wherein the periodic structurecausing optical diffraction is formed on a part or a whole of thesurface of the substrate.
 7. The method for forming a structural bodyaccording to claim 3, wherein the periodic structure causing opticaldiffraction is formed on a part or a whole of the surface of thesubstrate.
 8. The method for forming a structural body according toclaim 2, wherein the light comprises a plurality of light beamsintersected with each other, and the periodic intensity distributionchanges by changing a number, an intersecting direction, an intersectingangle, and a wavelength of the plurality of light beams to form theperiodic structure.
 9. An apparatus for forming a structural body forirradiating a substrate with light, comprising: a laser oscillatoradjusting irradiation pulse number and/or laser output such that aperiodic structure causing optical diffraction is formed on a surface ofthe substrate.